Over the past ten years, the inventors of the present invention have been active in researching, fabricating, and demonstrating millimeter wave holographic imaging techniques. In the course of this effort, they and others have published numerous patents and other publications that describe millimeter wave holographic imaging techniques generally, as well as specific improvements and enhancements. A review of this literature is useful to provide an understanding of the field, and exemplary publications are provided below. Each of these, together with any other patent or publication referenced herein, are hereby incorporated herein in their entirety by this reference.
D. M. Sheen, “Combined illumination cylindrical millimeter-wave imaging technique for concealed weapon detection,” Proceedings of the SPIE—Aerosense 2000: Passive Millimeter-wave Imaging Technology IV, vol. 4032, 2000.
D. M. Sheen, H. D. Collins, T. E. Hall, D. L. McMakin, R. P. Gribble, R. H. Severtsen, J. M. Prince and L. D. Reid, “Real-time wideband holographic surveillance system.” U.S. Pat. No. 5,557,283, 1996.
D. M. Sheen, D. L. McMakin, T. E. Hall, and R. H. Severtsen, “Real-time wideband cylindrical holographic surveillance system.” U.S. Pat. No. 5,859,609, 1999.
D. M. Sheen, D. L. McMakin, and T. E. Hall, “Cylindrical millimeter-wave imaging technique for concealed weapon detection,” Proceedings of the SPIE—26th AIPR Workshop: Exploiting new image sources and sensors, vol. 3240, pp. 242-250, 1997.
D. M. Sheen, D. L. McMakin, H. D. Collins, and T. E. Hall, “Near field millimeter-wave imaging for weapon detection,” Proceedings of the SPIE—Conference on Applications of Signal and Image Processing in Explosive Detection Systems, vol. 1824, pp. 223-233, 1992.
D. L. McMakin, D. M. Sheen, T. E. Hall, and R. H. Severtsen, “Cylindrical holographic radar camera,” Proceedings of the SPIE—The International Symposium on Enabling Technologies for Law Enforcement and Security, I. 3575, 1998.
D. L. McMakin, D. M. Sheen, H. D. Collins, T. E. Hall, and R. R. Smith, “Millimeter-wave high resolution holographic surveillance system,” Proceedings of the SPIE EUROPTO International Symposium on Substance Identification Technologies, vol. 2092, pp. 525-535, 1993.
D. L. McMakin, D. M. Sheen, H. D. Collins, T. E. Hall, and R. H. Severtsen, “Wideband, millimeter-wave, holographic weapons surveillance system,” Proceedings of the SPIE—EUROPTO European symposium on optics for environmental and public safety, vol. 2511, pp. 131-141, 1995.
D. L. McMakin, D. M. Sheen, and H. D. Collins, “Remote concealed weapons and explosive detection on people using millimeter-wave holography,” presented at 1996 IEEE International Carnahan Conference on Security Technology, 1996.
D. L. McMakin and D. M. Sheen, “Millimeter-wave high-resolution holographic surveillance systems,” presented at AAAE Airport Security Technology Conference, Atlantic City, N.J., 1994.
D. L. McMakin, R. H. Severtsen, T. E. Hall, and D. M. Sheen, “Interrogation of an object for dimensional and topographical information.” U.S. Pat. No. 6,703,964 B2.
Many of the near real-time imaging systems described in the aforementioned publications use linear arrays of microwave/millimeter wave antennas that are sequentially switched electronically to allow high-speed sampling along the array axis. Mechanical scanning, in a direction perpendicular to the array axis, then completes the sampling of a two dimensional aperture of wideband holographic image data. This data can then be reconstructed using wideband holographic imaging algorithms, typically using a computer configured to automate the process, resulting in a focused image.
A preferred wideband holographic imaging technique is described in detail in D. M. Sheen, D. L. McMakin, and T. E. Hall, “Three-dimensional millimeter-wave imaging for concealed weapon detection,” IEEE Transactions on Microwave Theory and Techniques, vol. 49, pp. 1581-92, 2001. A similar scanning technique can be employed in a cylindrical fashion using a linear array that is scanned over a circular path around the target to be imaged.
In these and other configurations, a major cost of these systems is the array of antennas. The most direct and obvious method of scanning along the array axis is to assume that each antenna is placed uniformly along the axis of the linear array, and can function simultaneously as a transmitter and receiver. This scenario is depicted in FIG. 1.
A switching network is used to sequentially select each antenna element and then use it to transmit and receive the wideband microwave/millimeter-wave signal. An antenna spacing of Δ results in an effective spatial sample spacing of Δ. While conceptually simple, this technique has a number of drawbacks. First, the antennas must be spaced very closely, usually on the order of one-half wavelength at the center frequency in order to satisfy the spatial sampling criterion on the aperture. This forces the antenna to be very small, and therefore low-gain, and will frequently cause antenna coupling problems between adjacent or neighboring antennas.
An additional problem is that the microwave/millimeter-wave transceiver must be capable of separating the transmit from the receive signal. This is possible using directional couplers or circulators, however, these introduce additional losses and do not perfectly isolate the weaker received signal from the much stronger transmitted signal.
Accordingly, there exists a need for improved methods and apparatus for wideband holographic imaging that minimizes the cost of these systems.